Very few of the fluids used in Hydraulic Systems are typically exceptional in all considerations; every application differs from the others, industrial and mobile equipment work under terribly extreme conditions. Now days, there is a big variety of fluids out there for every application.

All these are the most common types of fluids applied today.

1. Petroleum. Hydraulic petroleum-based oils are the most usually used stock for hydraulic applications where there is no danger of fire, no probability of leakage that may cause contamination of other products, no large temperature fluctuations, and no environmental impact.

2. Fire resistant. In applications in which fire hazards or environmental pollution are a problem, water based or aqueous fluids offer various advantages. The fluids includes of water-glycol and water-in-oil fluids utilizing emulsifiers, stabilizers, and additives. Due to their reduced lubricity,
piston pumps used with these fluids would be limited to 3000 psi.

3. Water-glycol. These fluids incorporate from 35 to 60 percent water to supply the fire resistance, in addition to glycol antifreeze such as ethylene, Di-ethylene, or propylene which is nontoxic and biodegradable, including a thickener such as poly-glycol to provide the required
viscosity. These types of fluids also provide all the important additives such as anti-wear, foam, rust, in addition to corrosion inhibitors. Viscosity, pH, and water hardness monitoring are extremely important in these systems.

4. Oil-in-water. These fluids are made from very small oil droplets dispersed in a continuous water phase. All these fluids have low viscosity, excellent fire-resistance, and good cooling ability due to the large Proportion of water. Additives must be useful to improve their inherently poor lubricity and to protect against corrosion.

5. Water-in-oil. The water content of hydraulic water-in-oil fluids could be approximately 40 percent. These fluids consist of very compact water droplets dispersed in a continuous oil phase. The oil phase offers good to excellent lubricity as the water content provides the needed level of fire-resistance and enhances the
fluid cooling ability.

6. Synthetic fire-resistant fluids. Three varieties of synthetic fire-resistant fluids are manufactured: phosphate esters, chlorinated, hydrocarbons, and also synthetic base. These fluids do not contain water or unstable materials, and they provide acceptable operation at high temperatures without losing essential elements. The fluids are also appropriate high-
pressure applications.

7. Vegetable oils. Hydraulic vegetable oils production reaches up to the billions of gallons in these days. However, due to technological complexity and monetary reasons, few are usable for formulating EA (environmentally acceptable) fluids. The useful hydraulic vegetable oils offer excellent lubricating capabilities and they are nontoxic and highly biodegradable, relatively more affordable compared to synthetic fluids, and are built from natural renewable resources.

8. Rapeseed oil, or canola oil, seems to be the base for the most common of the biodegradable hydraulic fluids. The high-quality of RO has improved over time, and it has become more and more popular, but it has issues at both high and low temperatures and is inclined to age rapidly. Its cost, approximately double that of mineral oil still causes it to be more affordable than many other EA fluids.

10. Poly-glycol. The application of these is declining due to their aquatic toxicity while mixed with lubricating additives and the incompatibility with mineral oils as well as seal materials. Poly-glycol fluids have been offered for several a long times and are widely used. They also are actually used since the mid-1980s in construction machines (excavators) and a variety of stationary installations. Those were the first biodegradable oils on the industry.

11. Water. By having the prospect of increasingly stringent environmental restrictions on the application mineral-oil based hydraulic fluids, water may become a realistic alternative. Pure water has low lubricity and cannot work as a
lubricant in the standard sense, but water has been utilized as hydraulic fluid in specialty usages where leakage contamination and fire hazard are primary concerns. New designs and also usage of positively wear-resistant materials have exposed possibilities for new water hydraulic utilization.